How to make colored backgrounds not influence the color of an object
When you are using any color (or image) as background in the world settings you have to imagine a shpere that completely surrounds the scene and that is emitting colored light from every point.



There are a few alternatives.


One is to have two different shaders for the background, and control them using a light path node, so that difuse rays are a neutral color and the rest are green.


You can use a neutral color (or no background at all) for the background and enable transparency (film transparent), and then add the background color in the compositor using alpha over.




I hope you are not putting green behind the object with the idea of using a chroma key later on to take away the background. If that is the case then just render with a transparent background and save in a format that can handle an alpha channel (EXR or TIFF).